The invention relates, in general, to coolant pumps for nuclear reactors. More particularly, the invention pertains to an improved gas circulator for a gas-cooled nuclear reactor.
Gas-cooled nuclear reactors are known. For instance, U.S. Pat. Nos. 3,138,535, 3,201,320, 3,201,321, all to Fortescue et al.; U.S. Pat. No. 3,244,598 to Rose et al.; U.S. Pat. No. 3,444,038 to Schabert; and commonly owned, copending application Ser. No. 810,993, entitled "Inherently Safe, Modular, High-Temperature Gas-Cooled Reactor System," describe gas-cooled nuclear reactors. The disclosures of such patents and patent application are hereby incorporated herein by reference.
A coolant pump for a gas-cooled nuclear reactor is generally referred to as a circulator or blower. Several types of circulators for gas-cooled nuclear reactors are known, e.g., electric drive circulators and turbine drive circulators.
Some older gas-cooled nuclear reactor systems employ electric drive circulators. The drive motor is outside of the primary system pressure boundary, while the impeller is inside of the primary system pressure boundary. Consequently, the drive shaft extends through the primary system pressure boundary, and a shaft seal for the drive shaft is required. The shaft seal for the drive shaft has reactor system pressure on one side of it and ambient pressure on the other side of it.
A gas-cooled nuclear reactor system with a system-to-ambient shaft seal has a number of disadvantages. Seal design constraints may limit reactor system operating pressures and/or temperatures. The heat transfer effectiveness of the gas coolant, therefore, may be adversely affected. Moreover, seal design constraints may limit the maximum rotational speed of the circulator, thereby constraining its design. Furthermore, sealing fluids may become a source of contamination for the reactor system.
Some newer gas-cooled nuclear reactor systems utilize electric drive circulators or turbine drive circulators that are located within the primary system pressure boundary. In other words, the circulator is submerged in the coolant gas pressure envelope. Consequently, only cable penetrations and/or pipe penetrations through the primary system pressure boundary are necessary. A nuclear reactor system with a submerged circulator eliminates a system-to-ambient shaft seal, and it may tolerate higher system pressures and/or temperatures than the older gas-cooled nuclear reactor systems. One of two variations of the submerged circulator concept is typically used in a gas-cooled nuclear reactor system. Specifically, a system may have electric drive circulators with oil-lubricated bearings or steam drive circulators with water-lubricated bearings.
However, several problems are associated with gas-cooled nuclear reactor systems having submerged circulators. Lubrication of the bearings for a submerged circulator may be burdensome because the bearings are contained within the primary system pressure boundary. A bearing lubrication system for a circulator may have a source of a bearing lubricant and/or a source of a buffer gas located outside of the primary system pressure boundary. Accordingly, the bearing lubrication system may be complex, expensive, and difficult to install and maintain. Furthermore, if oil or water is used as a lubricant, the oil or water lubricant may enter and contaminate the reactor system.
In gas-cooled nuclear reactor systems having electric drive circulators with oil-lubricated bearings, rapid depressurization of the reactor may disrupt the lubrication system and, therefore, result in a loss of the capability for forced circulation, and may cause oil to enter and contaminate the reactor system. In addition, the maximum rotational speed of the circulator may be limited by the so-called oil whip problem. Moreover, the maximum temperature of the gas coolant at the inlet of the circulator may be limited inasmuch as the temperature of the oil-lubricated bearings must be kept within the temperature limits of the oil.
In gas-cooled nuclear reactor systems having turbine drive circulators with water-lubricated bearings, rapid depressurization of the reactor may disrupt the lubrication system and cause water to enter the reactor system. Additionally, turbine drive circulators with water-lubricated bearings are typically designed so that the bearing water pressure is greater than the reactor system pressure, and a buffer gas is utilized to balance the bearing water pressure and the reactor system pressure. However, the bearing water pressure may overcome the buffer gas pressure at certain times. As a result, water leakage into the reactor system is a significant problem for turbine drive circulators with water-lubricated bearings.
In a circulator with conventional bearings, the rotor may become unbalanced and cause the circulator to vibrate, which results in metal fatigue and noise. The vibration-generated noise problem may be a crucial consideration in certain applications of a gas-cooled nuclear reactor system. Furthermore, if the rotor imbalance is excessive, the cost of correcting this situation is high since the rotor must be removed from the circulator and rebalanced.
Accordingly, a need exists for a circulator for a gas-cooled nuclear reactor that minimizes the potential for contamination of the reactor system, that imposes few constraints on reactor system operating parameters, and that minimizes vibrations.